Integrated circuit designers have always sought the ideal semiconductor memory: a device that can be randomly accessed, written or read very quickly, is non-volatile but indefinitely alterable, and consumes little power. Magnetic Random Access Memory (MRAM) technology has been increasingly viewed as offering many of these advantages.
An MRAM device typically includes an array of magnetic memory elements 11 located at the intersections of row line 13 and column line 15 conductors as illustrated in FIG. 1. A simple magnetic memory element has a structure which is shown in more detail in FIG. 2. The magnetic memory element includes ferromagnetic layers 17 and 19 separated by a nonmagnetic layer 21. The magnetization in one ferromagnetic layer 17, typically referred to as the pinned layer, is fixed in one direction. The magnetization of the other ferromagnetic layer 19, often referred to as the sense layer, is not pinned, so its magnetization is free to switch between “parallel” and “anti-parallel” states relative to the magnetization of the pinned layer. The sense layer may also be referred to as the “free” layer or storage layer, thus it should be understood that when the term “sense layer” is used, the meaning of this term should not be limited to this terminology but rather the function the layer performs.
The logical value or bit stored in an MRAM memory element is associated with a resistance value, and the resistance of the memory element is determined by the relative orientation of the sense layer magnetization with respect to the pinned layer magnetization orientation. A parallel orientation of the magnetization of the sense layer with respect to the pinned layer magnetization results in a low resistance. Conversely, in response to the anti-parallel orientation, the magnetic memory element will show a high resistance. Referring to FIG. 2, the manner in which the resistance of the memory element is read is dependent on the type of material used to form the nonmagnetic spacer layer 21 separating the pinned layer 17 from the sense layer 19. If the nonmagnetic spacer layer 21 is made from a conducting material, such as copper, then the resistance value of the memory element can be sensed via the giant magnetoresistance effect, which usually involves running a current parallel to the long axis 20 of the memory element. If the nonmagnetic spacer layer is composed of an insulating material, such as alumina, then the resistance value can be sensed using the tunneling magnetoresistance effect, and this is accomplished by running a current perpendicular to the plane of the nonmagnetic spacer layer 21 from the sense layer 19 to the pinned layer 17.
A logical “0” or “1” is usually written into the magnetic memory element by applying external magnetic fields (via an electrical current) that rotate the magnetization direction of the sense layer. Typically an MRAM memory element is designed so that the magnetization of the sense layer and the pinned layer aligns along an axis known as the easy axis 27. External magnetic fields are applied to flip the orientation of the sense layer along its easy axis to either the parallel or anti-parallel orientation with respect to the orientation of the magnetization of the pinned layer, depending on the logic state to be stored.
MRAM devices typically include an orthogonal array of row and column lines (electrical conductors) that are used to apply external magnetic fields to the magnetic memory elements during writing and may also be used to sense the resistance of a memory element during reading. Additional write and read conductors may also be present in the array. In the two conductor level implementation shown in FIG. 1, the magnetic memory elements are located at the intersections of the row 13 and column 15 lines.
Referring to FIG. 1, the magnetic field that is generated by running a current through the column line 15 is referred to herein as the easy-axis write field. The easy-axis write field is collinear with the easy axis 23 of the MRAM bit 11 (FIG. 2a). The magnetic field that is generated when a current is run through the row conductive line 13 is referred to as the hard-axis write field. The hard-axis write field generated by the row conductive line 13 runs parallel to the hard axis 25 of the MRAM bit 11.
A memory element is selected for writing when it is exposed to both a hard-axis and an easy-axis write field. Each write field, by itself and when generated with only one of the two conductive lines, is therefore commonly referred to as a half-select field because a single field by itself should not be of sufficient magnitude to switch the magnetization orientation of the sense layer of a memory element. In practice, however, the hard-axis write field is often referred to as the half-select field, while the easy-axis write field is often referred to as the switching field. These two fields are used to perform write operations on a specific memory element when applied in conjunction with each other by passing current through conductors 13, 15 (FIG. 1) intersecting at the selected element. The bit stored at the selected memory element being accessed for a read or write operation is referred to herein as a “selected bit”.
This method for selecting a bit for writing is not ideal. During a write operation, the unselected memory elements coupled to the particular column line 15 are exposed to the easy-axis write field. Similarly, the unselected memory elements 11 coupled to the particular row line 13 are exposed to the hard-axis write field. It is thus important to limit stray magnetic fields in the array of MRAM memory elements to a value that cannot cause half-selected bits to be written. Some sources of stray fields include fields from neighboring write conductors, stray fields emanating from the ferromagnetic layers of neighboring memory elements, and fields generated by sources external to the MRAM device. These stray fields may also inhibit a selected memory element from being written, if the combined value of the stray, hard-axis, and easy-axis fields is too small for a bit to be written. Another source of non-ideal behavior that manifests itself in the write current required to write a memory element results from the difficulty in making an array of MRAM memory elements that respond identically to the applied write fields. Some sources of this effect include variations in element-to-element geometry, variations in element-to-element magnetic properties, and thermally activated magnetization fluctuations. Therefore, the particular value of the hard and easy-axis write fields, and thus the row and column line write currents, is a compromise such that selected memory elements are selected with enough margin that they are always written and unselected memory elements are never exposed to a field large enough that they are unintentionally written.
Thermal effects, such as superparamagnetism or thermally activated magnetization reversal, and the effect of stray fields emanating from neighboring bits may cause problems in MRAM devices. Either of these of these mechanisms can result in unpredictable write and read behavior. Using a conventional single-layer sense layer, these effects will be extremely difficult to overcome as the bit density of the MRAM device is increased.
Thermal fluctuation in a seemingly unfluctuating macroscopic observable quantity, such as the magnetization of a ferromagnetic material, is an abstract concept. The orientation and magnitude of the magnetization of a ferromagnetic material are in actuality statistical quantities. In any material, fluctuations in thermal energy are continually occurring on a microscopic scale, where the magnitude of the thermal fluctuations is determined by the temperature T of the material. These fluctuations when averaged over the entire volume of the specimen in question determine the macroscopically observable property of the system. On a microscopic level, the probability for any atom in a ferromagnetic material to have a magnetic moment oriented in a particular direction is proportional to the Boltzmann factor, e−U/KbT. Here, U is the energy associated with, in this case, a particular magnetic moment orientation and Kb is Boltzman's constant. As the energy of a particular atomic moment orientation decreases, or as T and thus the thermal energy increases, the likelihood of an atom having a moment oriented in a particular direction for a fixed period of time decreases. If the ratio of U/KbT becomes small enough, the magnetic moment of the atom will spontaneously change direction under the influence of thermal fluctuations. The sense layer of an MRAM memory element may be thought of as a collection of atomic magnetic moments or spins that are tightly coupled together and aligned in the same direction. The energy required to orient the magnetization of the sense layer can thus be considered as the product of some energy density times the volume of the sense layer, Usw=UpV. As the temperature increases or as the sense layer volume decreases, the likelihood of the sense layer to be found in a particular orientation decreases. Again, if Usw/KbT is small, the orientation of the sense layer's magnetization is unstable to thermal fluctuations. Memory bits that are characterized by low values of Usw/KbT tend to have poor data retention times. Wait long enough and a fluctuation can spontaneously reverse the magnetization. A magnetic memory element that can spontaneously reverse its magnetization on a time scale short compared to the required data retention time is said to have hit the superparamagnetic limit. As the superparamagnetic limit is approached, magnetic memory elements also become more susceptible to half-select write events or the occasional inability to write a selected memory element, as a thermal fluctuation may inhibit a bit from writing in a specified time interval, or it may write a half-selected bit in a specified time interval. Note that increasing MRAM memory density necessitates a decrease in the volume of an MRAM bit and thus pushes the technology closer to the superparamagnetic limit or to the regime where thermal fluctuation becomes a problem.
Conventional MRAM memory elements also suffer from inter-bit interactions resulting from stray magnetic fields that emanate from the magnetic memory element 11 sense layers. The sense layer of an MRAM memory element 11 produces stray fields because it is not a closed magnetic flux structure. It thus forms poles at the edges of the sense layer in response to the orientation of the sense layer magnetization. The orientation of the stray field therefore changes with the orientation of the magnetization of a sense layer in a memory element. The stray fields produced by these poles decrease in value with increasing distance from the sense layer. At any memory element in an MRAM array, the stray fields from the neighboring bits may, however, be significant compared to the field required to switch the bit. In future MRAM designs, memory elements will need to be packed together more closely, which will compound this problem. A need therefore exists to reduce the stray fields produced by the magnetic layers in an MRAM memory element and reduce memory element sensitivity to thermal fluctuations.